125 a sub panel

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Old 09-03-14, 04:57 PM
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125 a sub panel

Our oil fired tank less hot water system failed. I have purchased an eco smart tank less 27kv electric hot water heater. Our main circuit box has 200A service, but not enough space for the 3 40A breakers (2 pole) that the system requires. I want to install a 125A sub panel with the 3 40A breakers. What size wire do I need to run form the main box to the sub box that will be protected by a 125A breaker. It will be about a 25 foot run.

Thanks,

Kevin
 
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Old 09-03-14, 05:12 PM
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Welcome to the forums.

It's a shame the tankless heater is so far from the main panel. You would have been better off connecting the tankless heater right to the main panel and shed some of those circuits to the sub panel.

Your 125A sub panel is right at the edge of being too small. I consider that 27K load as a continuous load which means you need a supply of 135A.

I would use #1 wire as the supply wire. Are you going to run a conduit or do you need a cable size designation ?
 
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Old 09-03-14, 06:17 PM
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I highly doubt that a tank-less water heater will be running for 3 hours or more. I would not consider it a continuous load.

If you run conduit, you can use #2 copper. If you use an NM-b cable (copper) you need 1/0.
IF you want to use aluminum cable you need 2/0. If its XHHW AL you need 1/0.
You need to use a #6 copper, or #4 Aluminum for a ground.
 
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Old 09-03-14, 07:26 PM
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Our main circuit box has 200A service, but not enough space for the 3 40A breakers (2 pole) that the system requires.
What is your existing calculated load? You may or may not have enough capacity to add 27 KW load to the 200 amp service. The possibility exists that you may need a 400 amp service.
 
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Old 09-03-14, 09:45 PM
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I would advise you to have a load calculation done and also talk to your power company before attempting to add that much load.
 
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Old 09-04-14, 08:03 AM
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I would advise you to have a load calculation done and also talk to your power company before attempting to add that much load.
That is an excellent point by PCboss. You may have a 200 amp service, but the power company may not have the infrastructure in place for that much additional load. The service drop and maybe even the transformer might have to be changed. Most power companies size their drops and transformers by the actual load. It is usually the customer's responsbility to inform the power company of any new additional loads on their system.
 
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